Polymorphism is a phenomenon which has invited questions from geneticists for decades. Mutations causing varying phenotypes at specific loci within a species are the foundation of evolution; but once they arrive in a population, unless they are lucky or bring a distinct benefit to the organism they will be lost. Allele frequency within a population can be affected by several factors. If there are no selective pressures on individuals of different phenotypes, stochastic changes – genetic drift – will lead alleles randomly towards fixation or loss in different populations. This occurs at a much faster rate in small populations produced by population bottlenecks and founder events. If there are selective pressures genetic drift will still occur, but will be resisted in favour of the beneficial allele, leading to fixation of the beneficial allele with time.
Cepaea nemoralis is a species of snail which shows large amounts of polymorphism in the genes which contribute to shell colour and the presence and patterns of bands. According to previous studies (Jones et. al, 1977) the genes which control colour and banding are linked together in a supergene; a group of genes found together on a chromosome with no or little recombination between them. The presence of polymorphism exists in populations all over Europe, suggesting that, despite genetic drift the polymorphism is maintained as a result of variable, separate factors which favour different morphs. In previous studies scientists have been trying to find one conclusive explanation for the persistence of polymorphism in C.nemoralis. When it is understood how polymorphism is maintained within C.nemoralis this knowledge can then be applied to other species.
On its own, genetic drift leads to fixation over time. However, a computer simulation shows that gene flow can maintain the polymorphism for many generations, by moving alleles around from distinctly different populations, as long as the migration rate is sufficiently low. With migration rates between 0.1 and 0.01, which is expected in snails, the polymorphism could be maintained for over 10000 generations. While genetic drift will lead to fixation and can be resisted by migration, selection on various characteristics can impact the frequency of alleles.
Visual Selection is applied to C. nemoralis by predators such as thrushes, glow worms, predatory snails and beetles. These predators may change allele frequency in favour of phenotypes which are best camouflaged; for example darker colours expressed in the woods and light colours with stripes in grassy areas. It is difficult to determine the selective intensity of predation as snails will be carried away from their original population to an anvil stone by thrushes to be broken open. Selection by predators has been demonstrated to be frequency-dependent; whereby rarer morphs are favoured. This is because predators which hunt by sight, for example thrushes form a “searching image” of the first prey encounter in a location. They then continue to hunt for this morph at the exclusion of others. This will favour rare morphs as they are unlikely to be the first morph sampled. Rare morphs will continue to be advantageous until a frequency dependent equilibrium is reached. This is known as apostatic selection. C. Nemoralis could also be affected by climatic selection, which selects individuals on their ability to absorb solar radiation. This is an important factor to C. nemoralis as it is ectothermic. In dark environments like the woods brown-colour alleles will be selected for as they will heat up faster and be more active than other morphs . (Jones et al, 1977). This is important as individuals may find themselves close to their lethal limit in a cold environment. In a study, varying forms of caged C. Nemoralis were placed in an area known to have a cold climate. It was found that brown snails survived longer than other forms. Polymorphism could feasibly be maintained where environments are varied. If the selective pressures are different in each environment, populations will have a tendency to acquire those characteristics. However, a small amount of gene flow will result in each of these populations having a small minority of unfavourable morphs.
We investigated the causes of polymorphism by sampling the snails from three different habitats; woodland, scrubland and grassland. In addition, we took samples from varying depths of these environments, on a scale from dense forest to wide open field, in different locations to compare with each other. By taking note of these locations and their geographic location, we can establish two things. If there appears to be a gradient of frequency whereby, regardless of geography, there are always more brown snails than yellow ones in the forest, and there are gradually more yellow snails farther into the field, it is likely that selection is at play. If, however, the frequency of phenotype appears to be either random or have a gradient which seems to centre around one location geographically, it is more likely that the prevalence is due to genetic drift and small population sizes and the selective pressure is minor.
Discussion
Our investigation has clearly demonstrated an abundance of snails with darker shells (either brown in colour, or demonstrating heavy or fused banding) in the shaded, wooded areas, contrasted with a scarcity in the open grassland. This would suggest some sort of selective pressure, either visual - by camoflage from predating thrushes - or climatic - with darker snails being able to heat and become active better than the lighter ones. This pattern is consistent between woodland sites on all sides of the field.
However, within the field (which is a mixture of open grassland and scrub) there does not appear to be a consistent pattern between habitats. Moreover, it appears that abundances of certain colours of snail appear to vary randomly, with each individual population appearing more similar to its direct neighbours than to other sites farther away which may be more similar in terms of habitat type. This is strong support for the idea that the polymorphism, at least within the field areas, is perpetuated by a mixture of genetic drift and some gene flow between adjacent populations.
Whilst most values for specific ‘habitat types’ were calculated as sums of close samples (shown as insignificantly different by chi-square tests), one particular sample was radically different from one only ten metres away. Chi square tests suggest that this is not due to sampling error but may be explained by a recent founder event by one or two individuals and the consequent increased power of genetic drift.
An interesting observation in this particular field is that whilst there is in general, a prevalence of ‘dark’ shells in the woods, these come in two forms. On the woods on the east side of the field, most of the dark shells are simply brown in colour and demonstrate little or no banding. On the west side, however, there is almost complete loss of the allele for brown colour, but instead a great number of the snails have bands that have fused almost to the extent of making the entire shell appear very dark. This may suggest that if the brown allele was lost due to genetic drift (perhaps as populations of snails crossed the field) as they started to repopulate the woods a selective pressure of some description favoured those snails with more banding as a compensatory characteristic.
Our results fit with those of Clarke (1960) and Ozgo (2011), which suggests that a visual or climatic selective pressure is driving the snails in dark areas to express brown colouration or banding. Alternatively, our odd discovery of the loss of the brown colour allele despite the two areas of wood being connected may suggest a third selective pressure which is not necessarily detectable by human experimenters without specialist examination, which has not yet been considered.
Obtaining of accurate data was not helped by the tendency of other people at the site to pick up specimens and take them to other areas for their own convenience. Whilst the best effort was taken to sample from areas which appeared undisturbed, there may be some undetected bias. In general, however, data seems relatively reliable, and the use of many different sample sites for each habitat type adds weight to the claim that there is indeed selective pressure within the woodland for snails to exhibit darker shells. Unfortunately, as some of our data seems inconclusive and we sampled only from the north and west sides of the forest, we cannot state categorically that these patterns are maintained universally. In addition, more data with more accurate spatial measurements from the field may provide neater evidence for the genetic drift across the site. Further examination using these data as a pilot would be worthwhile.
*Please note that we've updated the introduction a fair bit since we posted it, and are still waiting on some more statistics to better define our results. It's all a little incoherent at the moment, but it won't be for long.
Lucy Wyatt*
Aamna Mohdin
Dominic Barker
Nisha Bargota
Priyanka Bulsara
Jenni Toes
Introduction
Polymorphism is a phenomenon which has invited questions from geneticists for decades. Mutations causing varying phenotypes at specific loci within a species are the foundation of evolution; but once they arrive in a population, unless they are lucky or bring a distinct benefit to the organism they will be lost. Allele frequency within a population can be affected by several factors. If there are no selective pressures on individuals of different phenotypes, stochastic changes – genetic drift – will lead alleles randomly towards fixation or loss in different populations. This occurs at a much faster rate in small populations produced by population bottlenecks and founder events. If there are selective pressures genetic drift will still occur, but will be resisted in favour of the beneficial allele, leading to fixation of the beneficial allele with time.
Cepaea nemoralis is a species of snail which shows large amounts of polymorphism in the genes which contribute to shell colour and the presence and patterns of bands. According to previous studies (Jones et. al, 1977) the genes which control colour and banding are linked together in a supergene; a group of genes found together on a chromosome with no or little recombination between them. The presence of polymorphism exists in populations all over Europe, suggesting that, despite genetic drift the polymorphism is maintained as a result of variable, separate factors which favour different morphs. In previous studies scientists have been trying to find one conclusive explanation for the persistence of polymorphism in C.nemoralis. When it is understood how polymorphism is maintained within C.nemoralis this knowledge can then be applied to other species.
On its own, genetic drift leads to fixation over time. However, a computer simulation shows that gene flow can maintain the polymorphism for many generations, by moving alleles around from distinctly different populations, as long as the migration rate is sufficiently low. With migration rates between 0.1 and 0.01, which is expected in snails, the polymorphism could be maintained for over 10000 generations.
While genetic drift will lead to fixation and can be resisted by migration, selection on various characteristics can impact the frequency of alleles.
Visual Selection is applied to C. nemoralis by predators such as thrushes, glow worms, predatory snails and beetles. These predators may change allele frequency in favour of phenotypes which are best camouflaged; for example darker colours expressed in the woods and light colours with stripes in grassy areas. It is difficult to determine the selective intensity of predation as snails will be carried away from their original population to an anvil stone by thrushes to be broken open. Selection by predators has been demonstrated to be frequency-dependent; whereby rarer morphs are favoured. This is because predators which hunt by sight, for example thrushes form a “searching image” of the first prey encounter in a location. They then continue to hunt for this morph at the exclusion of others. This will favour rare morphs as they are unlikely to be the first morph sampled. Rare morphs will continue to be advantageous until a frequency dependent equilibrium is reached. This is known as apostatic selection.
C. Nemoralis could also be affected by climatic selection, which selects individuals on their ability to absorb solar radiation. This is an important factor to C. nemoralis as it is ectothermic. In dark environments like the woods brown-colour alleles will be selected for as they will heat up faster and be more active than other morphs . (Jones et al, 1977). This is important as individuals may find themselves close to their lethal limit in a cold environment. In a study, varying forms of caged C. Nemoralis were placed in an area known to have a cold climate. It was found that brown snails survived longer than other forms.
Polymorphism could feasibly be maintained where environments are varied. If the selective pressures are different in each environment, populations will have a tendency to acquire those characteristics. However, a small amount of gene flow will result in each of these populations having a small minority of unfavourable morphs.
We investigated the causes of polymorphism by sampling the snails from three different habitats; woodland, scrubland and grassland. In addition, we took samples from varying depths of these environments, on a scale from dense forest to wide open field, in different locations to compare with each other. By taking note of these locations and their geographic location, we can establish two things. If there appears to be a gradient of frequency whereby, regardless of geography, there are always more brown snails than yellow ones in the forest, and there are gradually more yellow snails farther into the field, it is likely that selection is at play. If, however, the frequency of phenotype appears to be either random or have a gradient which seems to centre around one location geographically, it is more likely that the prevalence is due to genetic drift and small population sizes and the selective pressure is minor.
Discussion
Our investigation has clearly demonstrated an abundance of snails with darker shells (either brown in colour, or demonstrating heavy or fused banding) in the shaded, wooded areas, contrasted with a scarcity in the open grassland. This would suggest some sort of selective pressure, either visual - by camoflage from predating thrushes - or climatic - with darker snails being able to heat and become active better than the lighter ones. This pattern is consistent between woodland sites on all sides of the field.
However, within the field (which is a mixture of open grassland and scrub) there does not appear to be a consistent pattern between habitats. Moreover, it appears that abundances of certain colours of snail appear to vary randomly, with each individual population appearing more similar to its direct neighbours than to other sites farther away which may be more similar in terms of habitat type. This is strong support for the idea that the polymorphism, at least within the field areas, is perpetuated by a mixture of genetic drift and some gene flow between adjacent populations.
Whilst most values for specific ‘habitat types’ were calculated as sums of close samples (shown as insignificantly different by chi-square tests), one particular sample was radically different from one only ten metres away. Chi square tests suggest that this is not due to sampling error but may be explained by a recent founder event by one or two individuals and the consequent increased power of genetic drift.
An interesting observation in this particular field is that whilst there is in general, a prevalence of ‘dark’ shells in the woods, these come in two forms. On the woods on the east side of the field, most of the dark shells are simply brown in colour and demonstrate little or no banding. On the west side, however, there is almost complete loss of the allele for brown colour, but instead a great number of the snails have bands that have fused almost to the extent of making the entire shell appear very dark. This may suggest that if the brown allele was lost due to genetic drift (perhaps as populations of snails crossed the field) as they started to repopulate the woods a selective pressure of some description favoured those snails with more banding as a compensatory characteristic.
Our results fit with those of Clarke (1960) and Ozgo (2011), which suggests that a visual or climatic selective pressure is driving the snails in dark areas to express brown colouration or banding. Alternatively, our odd discovery of the loss of the brown colour allele despite the two areas of wood being connected may suggest a third selective pressure which is not necessarily detectable by human experimenters without specialist examination, which has not yet been considered.
Obtaining of accurate data was not helped by the tendency of other people at the site to pick up specimens and take them to other areas for their own convenience. Whilst the best effort was taken to sample from areas which appeared undisturbed, there may be some undetected bias. In general, however, data seems relatively reliable, and the use of many different sample sites for each habitat type adds weight to the claim that there is indeed selective pressure within the woodland for snails to exhibit darker shells. Unfortunately, as some of our data seems inconclusive and we sampled only from the north and west sides of the forest, we cannot state categorically that these patterns are maintained universally. In addition, more data with more accurate spatial measurements from the field may provide neater evidence for the genetic drift across the site. Further examination using these data as a pilot would be worthwhile.
*Please note that we've updated the introduction a fair bit since we posted it, and are still waiting on some more statistics to better define our results. It's all a little incoherent at the moment, but it won't be for long.